In recent years, wireless traffic is continuously increasing, and demand for frequencies is continuously increasing. An example of means for effectively using finite frequencies includes a cognitive radio technique. The cognitive radio technique is a technique of performing communication by recognizing a surrounding radio wave environment and by appropriately changing the frequency and mode. An example of the cognitive radio technique includes a white space cognitive radio technique of performing communication by finding a white space (White Space: WS), which is an unused band in frequency bands, according to time and place, without interfering wireless signals from systems that preferentially use each of the frequency bands. For example, rules related to the use of communication of TV white space (Television White Space: TVWS) are released in September, 2010 in the U.S.A.
In the white space cognitive radio technique, a system that has priority to use a frequency is called a primary system or a first system. A system that finds and uses a white space of the frequency band allocated to the primary system is called a secondary system or a second system. Examples of a method of finding the white space include a sensing mode and a database access mode.
In the sensing mode, a communication apparatus of the secondary system uses a sensing device to physically sense the white space. Examples of a sensing method of a wireless signal by the communication apparatus of the secondary system include energy detection, cyclostationary detection, matched filter, eigenvalue analysis, and the like. A frequency band in which the detection result in these methods is equal to or smaller than a reference threshold is determined as a white space.
In the database access mode, the communication apparatus of the secondary system accesses a database on a network to acquire information of the white space. The database stores information of the white space calculated from information, such as location of a transmission station of the primary system, transmission power, and transmission frequency, in association with location information. For example, the communication apparatus of the secondary system that uses the database access mode accesses the database once or more a day to use the white space.